Investigation of biological characteristics of fruit development and physiological disorders of Musang King durian (Durio zibethinus Murr.)

2.1.1 Sample selection

Samples were randomly selected from 6 to 7-year-old Musang King durian trees, ensuring uniformity in health, absence of pests and diseases, and consistent cultivation conditions.

2.1.2 Flowering characteristics

Flowering was monitored by marking 10 flowers per tree and recording flowering events every 4 h (at 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00). Parameters recorded included flower bud opening, petal separation, full petal opening, stamen and pollen development, pistil maturation, nectar secretion, and pollination completion. A total of 200 flowers across 20 trees were observed to determine the duration of each flowering stage.

2.1.3 Pollination and fruit set

Farmers manually pollinated flowers using pollen collected from Ri-6 durian flowers blooming at the same time. Pollen was applied with a nylon brush between 18:00 and 20:00. The fruit set rate was calculated as the proportion of pollinated flowers that developed into fruits. Observations continued until the ovary began to develop or dried up and dropped.

2.1.4 Fruit drop

Immature fruit drop was monitored by tagging 200 fruits across 20 trees and recording dropped fruits weekly from the fruit set until the cessation of fruit drop. The percentage of dropped fruits was calculated based on the total number of observed fruits.

2.1.5 Fruit development

Fruit development was assessed every 14 days by measuring fruit weight, length, and width, as well as the weights of the fruit rind, flesh, and seeds. Three fruits were sampled per tree during each measurement interval, resulting in 60 fruits per sampling period. At harvest (90 days after fruit set (DAFS)), fruit samples were analyzed for average weight, flesh yield (percentage of total weight), and quality parameters, including °Brix, total soluble solids (TSS), total sugar content, starch content, and water content.

2.1.6 Brix measurement (%)

A 0.5 g sample was ground with 1 mL distilled water and centrifuged using a HETTICH EBA 21 centrifuge at 3,500 rpm for 5 min. One drop of the supernatant was measured using an ATAGO refractometer (ATAGO Co., Ltd., Japan). The result was multiplied by three to obtain the final value.

2.1.7 TSS, °Brix

The measurement of TSS was carried out using a Brix refractometer according to DuBois et al. [12]. Approximately 2 g of the fruit sample was finely ground and diluted to 25 mL with distilled water in a volumetric flask. A 0.5 mL aliquot of the diluted sample solution was mixed with 0.5 mL of 5% polyethylene glycol solution and centrifuged for 2–3 min at 3,500 rpm to obtain a clear supernatant. The °Brix value was then measured using the refractometer.

To calculate TSS, the °Brix value of the blank sample was subtracted from the measured °Brix of the sample, and the result was adjusted for dilution as per the formula: TSS (°Brix) = °Brix (read) – °Brix (blank) × (1 + V/W), where TSS is the total soluble solids, V is the volume of water used for dilution (mL), and W is the weight of the fruit sample (g).

2.1.8 Total sugar content

Total sugar was extracted and measured using the phenol-sulfuric acid method [12]. Specifically, 0.3 g of the sample was mixed with 10 mL of 90% methanol and heated in a water bath at 70–99°C to extract sugars. The extraction process was repeated twice with 80% methanol. The combined extracts were filtered and diluted to 50 mL. A 1 mL aliquot was further diluted to 50 mL with distilled water for a colorimetric reaction with 5% phenol and concentrated sulfuric acid in a ratio of 1:1:5. Absorbance was measured at 490 nm using a Shimadzu UV-1201 V spectrophotometer. Total sugar content was calculated based on a glucose standard curve using the formula:

where a is the measured sugar content (mg), b is the control sugar content (mg), V₁ is the volume of sugar extract (mL), v is the volume of diluted solution for measurement (mL), hspl is the dilution factor, W is the sample weight (g), 1,000 is the conversion factor from g to mg.

2.1.9 Starch content

Starch content was determined using the method described by Coomibs et al. [13]. Samples were dried at 60–70°C for 30 min after sugar extraction. Subsequently, 5 mL of distilled water was added, and the mixture was heated in a water bath for 15 min. After cooling, 2 mL of 9.2 N perchloric acid was added and stirred for 15 min. The solution was diluted to 10 mL and centrifuged at 4,000 rpm for 3 min to collect supernatant (1). The residue was treated again with 2 mL of 4.6 N perchloric acid, stirred for 15 min, diluted to 10 mL, and centrifuged to obtain supernatant (2). The combined supernatants were used to quantify glucose. A 5 mL aliquot of starch extract was diluted to 50 mL with distilled water. A 5 mL portion of this solution was mixed with 10 mL of anthrone reagent and heated in a water bath for 7.5 min. After rapid cooling in an ice bath, absorbance was measured at 630 nm. Starch content was calculated using the formula:

where a is the measured starch content (mg), b is the control starch content (mg), V₁ is the volume of starch extract (mL), v is the volume of diluted solution for measurement (mL), hspl is the dilution factor (5 mL extract/100 mL distilled water), W is the sample weight (g), and 1,000 is the conversion factor from g to mg.

2.1.10 Physiological disorders

Physiological disorders such as hardened flesh, discoloration, and aril tip burn were recorded at 70 and 90 DAFS. Affected fruits were categorized, and the percentage of segments showing symptoms was calculated. Disorders were linked to potential causes such as environmental stress or nutritional imbalances.

2.1.11 Agronomic management

The fertilization schedule was designed to support optimal fruit development and minimize physiological disorders in Musang King durian. Fertilizer was applied four times during the fruit development stage. At 30 DAFS, each tree received 1 kg of NPK fertilizer with a 15:15:15 formula to provide a balanced supply of essential nutrients for early fruit growth. Subsequently, at 45 DAFS, 1 kg of NPK fertilizer with a 30:10:10 formula was applied to enhance nitrogen availability during the phase of rapid fruit development. At 60 DAFS, 0.5 kg of NPK fertilizer with a 20:10:10 formula was provided to maintain steady growth as the fruit flesh began forming. Finally, at 75 DAFS, 0.5 kg of NPK fertilizer with an 11:12:18 formula was applied to support fruit ripening and quality improvement, particularly for enhancing phosphorus and potassium levels.

Irrigation was carried out every other day in the morning throughout the fruit development period to ensure sufficient moisture availability and prevent water stress. The volume of water supplied was adjusted based on the developmental stage of the fruit. From fruit set to 14 DAFS, 100 L of water were provided per tree during each irrigation cycle to support initial fruit formation. From 15 to 70 DAFS, the water volume was increased to 200 L per tree to accommodate the rapid growth phase when water demand is highest. In the final stage, from 71 to 90 DAFS, the water volume was reduced back to 100 L per tree to prevent overhydration and reduce the risk of physiological disorders, such as hardened flesh and aril tip burn.

This comprehensive fertilization and irrigation management strategy was tailored to meet the dynamic nutrient and water requirements of Musang King durian during its fruit development cycle. By aligning nutrient supply and moisture availability with critical growth stages, the approach aimed to optimize fruit yield and quality while mitigating the occurrence of physiological disorders.

2.1.12 Flower induction

Flowering was induced in the favorable season from October 2023 to July 2024, following the third shoot topping. Paclobutrazol at a concentration of 1,250 ppm was applied uniformly to each tree. The solution was sprayed to wet both the inner and outer surfaces of the canopy leaves, using 10 L of solution per tree. Applications were conducted early in the morning under cool conditions, avoiding direct sunlight.

To enhance flower bud induction, MKP (0-52-34) was applied twice at a concentration of 0.5%, seven and 14 days before flowering. Due to favorable weather, particularly a prolonged dry period from January 25, 2024, to February 15, 2024, flowering conditions were ideal. Flower buds began to emerge on February 10, 2024, with a uniform flowering pattern and no dormancy, achieving a flowering rate of 90%.

3.2.1 Fruit set

The fruit set rate of Musang King durian was recorded at 93%, which is notably high compared to other durian cultivars. This success is attributed to the combination of supplementary manual pollination and favorable environmental conditions. The compatibility between Musang King flowers and Ri-6 pollen further enhanced fertilization efficiency. Manual pollination ensured a controlled transfer of pollen, overcoming challenges posed by the declining population of natural pollinators. These findings align with previous research indicating that manual pollination improves fruit set by ensuring fertilization during the narrow window of floral receptivity [15,16].

Several factors influenced the high fruit set observed in this study. Precise timing during the peak flowering period, characterized by maximum stigma receptivity and pollen viability, was critical. Environmental conditions, such as temperature and relative humidity, also played a role, as excessive heat or prolonged wetness can negatively impact pollination success. For instance, optimal humidity levels during the late afternoon facilitated pollen adherence and fertilization. These results highlight the importance of integrating agronomic practices with environmental monitoring to achieve high fruit set rates.

Comparatively, Ri-6 durian in Tien Giang achieved an average fruit set rate of 86% under similar conditions [15]. The slightly higher performance of Musang King may be due to its floral morphology, which better supports manual pollination. Future studies should investigate whether genetic factors or specific pollination techniques further contribute to this advantage (Figure 4).

Figure 4

Musang King durian from pollination to fruit set stage: (a) durian fruit after pollination, (b) durian fruit after fertilization, developing ovary, and (c) unfertilized durian fruit. Source: Created by the author.

3.2.2 Immature fruit drop

Immature fruit drop was most pronounced during the first 14 DAFS, with a cumulative drop rate of 35% (Figure 5). This early-stage drop was primarily attributed to incomplete pollination, limited resource allocation, and environmental stress. By 42 DAFS, the fruit drop rate stabilized at a total of 48.5%, consistent with the natural thinning processes observed in other tropical fruit species [21,22]. Additionally, vegetative growth during fruit development, such as the emergence of new shoots, may have contributed to resource competition, further exacerbating fruit drop [16]. The timing of fruit drop reflects competition among developing fruits for resources such as carbohydrates and water. During the rapid growth phase, insufficient availability of these resources often triggers the abscission of weaker fruits. This process is regulated by hormonal signals, particularly a reduction in auxin levels and an increase in ethylene production in stressed or underdeveloped fruits [23,24]. Addressing these imbalances through targeted agronomic interventions, such as balanced fertilization and consistent irrigation, significantly reduced drop rates in this study.

Figure 5

Percentage of immature fruit drop from fruit set to harvest of Musang King durian in Phong Dien District, Can Tho City.

Agronomic practices implemented during the study period played a key role in minimizing excessive fruit drop. Regular irrigation tailored to the developmental needs of the fruit ensured adequate water supply, preventing stress-induced abscission. The application of NPK 30:10:10 fertilizer at 45 DAFS provided essential nutrients during the period of rapid fruit development, supporting retention. These findings corroborate previous research emphasizing the importance of nutrient and water management in mitigating fruit drop [25,26].

The high fruit set rate observed in Musang King durian underscores the effectiveness of manual pollination, particularly when conducted during the peak flowering period. This practice is vital for cultivars that exhibit asynchronous flowering or rely heavily on cross-pollination for reproductive success [27,28]. The results suggest that pairing Musang King with complementary varieties, such as Ri-6, can further enhance fruit set by ensuring consistent pollen availability. The significant reduction in fruit drop achieved through resource optimization highlights the importance of early intervention. Addressing key factors such as pollination success, hormonal regulation, and resource competition can substantially improve fruit retention. Future studies should explore the molecular mechanisms underlying fruit abscission to develop more targeted solutions.

Finally, the integration of environmental monitoring with agronomic management offers a promising strategy for improving both fruit set and retention. Precision agriculture tools, such as automated irrigation systems and real-time nutrient monitoring, could further optimize resource allocation, ensuring sustainable durian production in the face of climatic variability.

3.3.1 Fruit size

The growth of Musang King durian fruit followed a typical sigmoid curve, with distinct phases of development (Figure 6). Fruit size increased gradually during the first 28 DAFS, corresponding to the slow growth phase. Between 28 and 70 DAFS, the fruit entered a rapid growth phase, during which significant increases in both length and width were observed (Figure 6b). By 70 DAFS, fruit size reached near-maximum dimensions, with further increases tapering off as the fruit transitioned into the ripening phase.

Figure 6

Growth (a) and growth rate (b) of Musang King durian fruit size in Phong Dien District, Can Tho City.

Initially, the growth in length outpaced width, particularly between 14 and 42 DAFS (Figure 6a). However, from 56 DAFS onward, the growth in width exceeded that of length, contributing to the characteristic round to elliptical shape of Musang King durian. At harvest (90 DAFS), the average fruit size was 22.8 ± 0.8 cm in length and 21.1 ± 1.1 cm in width. These findings align with earlier studies on other durian varieties, such as Monthong and Ri-6, which exhibit similar growth patterns and size metrics [15].

3.3.2 Fruit weight

The weight development of Musang King durian follows a typical sigmoid growth curve with three distinct phases. From 0 to 28 DAFS, growth was slow, primarily involving organ formation. Between 28 and 70 DAFS, the fruit underwent rapid growth, characterized by the development of the rind, flesh, and seeds. Growth slowed again during the ripening phase, from 70 DAFS until harvest (Figure 7). The most significant weight gain occurred between 42 and 56 DAFS, driven by rapid flesh development, which peaked at a growth rate of 219.8 g per 2 weeks. Seed development also peaked during this period at 85.2 g per 2 weeks, while the rind continued growing until 70 DAFS, with a maximum rate of 571.7 g per 2 weeks. According to Hau et al. [15] on Ri-6 durian and Hau et al. [4] on Bi Ro hat lep durian, similar significant growth rates were observed between 42 and 56 DAFS.

Figure 7

Growth in weight (a) and growth rate (b) of Musang King durian fruit components from fruit set to harvest in Phong Dien District, Can Tho City.

Sapii and Nanthachai [29] described durian fruit development as occurring in three distinct periods: slow growth during the first 4 weeks, rapid growth from weeks 5 to 11, followed by slow growth until week 14, and minimal changes in size thereafter until harvest. The findings from this study align with these observations. At harvest (90 DAFS), Musang King durian fruit had an average weight of 2,161.8 ± 74.7 g, with the husk contributing 70.1%, the flesh 24.8%, and the seeds 5.2% of the total weight. These proportions align with observations from other durian cultivars, such as Ri-6, which typically weighs between 2.5 and 3.0 kg [4,15].

3.3.3 Fruit size and weight dynamics

The results highlight the interrelation between fruit size and weight dynamics in Musang King durian. The observed sigmoid growth pattern for both metrics reflects common physiological processes in tropical fruits, driven by cell division during the early phase, followed by rapid cell expansion and accumulation of reserves in later phases. The initial dominance of length growth followed by width expansion contributes to the characteristic shape of Musang King durian, which may influence consumer preferences [30]. Comparatively, the size and weight dynamics of Musang King durian align closely with those reported for Monthong and Ri-6 durians, with minor variations attributable to genetic differences and environmental factors [4,30]. For instance, Musang King’s smaller size but comparable weight highlights its higher density, which may affect texture and marketability. The rapid growth observed between 42 and 56 DAFS corresponds to the period of highest nutrient demand, emphasizing the need for targeted nutrient application and irrigation during this critical phase.

The uniformity in growth phases across trees suggests that Musang King durian is well-suited for synchronized agronomic practices. Stage-specific interventions, such as NPK 30:10:10 during rapid growth and consistent irrigation, can optimize both size and weight while reducing the risk of physiological disorders. Additionally, the observed rind-to-flesh ratio, while consistent with other varieties, underscores the importance of further research into increasing edible yield without compromising fruit protection [15,17].

The results of the survey on flowering treatment, fruit set, and fruit development are summarized in Table 1. The findings demonstrate that Musang King durian trees exhibit consistent reproductive processes, with minimal variation in timing among trees. The total duration from flower bud emergence to harvest was 149 days, including 90 days from fruit set to harvest. This uniformity in phenological stages suggests that the flowering and fruiting patterns of Musang King durian can be effectively predicted and managed under similar environmental and agronomic conditions.